The present invention relates to infrared imaging and, more particularly, to a sensing material array for imaging infrared radiation in two different bands, and to a focal plane array based on this sensing material array.
Many applications require the ability to image a scene at infrared wavelengths in two different spectral bands, a capability that is commonly called "dual color thermal imaging". These applications include remote sensing for temperature determination and pollution monitoring.
Several types of dual color detectors are known. Borrello et al., in U.S. Pat. No. 4,956,686, teach a sensing material array of parallel columns of pixels based on two different HgCdTe alloys. Forrest et al., in U.S. Pat. No. 5,518,934, teach a sensing material array, based on a stack of three QWIPs (quantum well infrared photodetectors) that are sensitive to successively shorter wavelength bands. Each pixel of the array includes apertures that expose one of the three QWIPs to infrared radiation incident on that QWIP. Photodetectors that are based on two QWIPs in tandem, meaning that one QWIP is directly behind the other relative to the direction of propagation of incident radiation, so that incident radiation impinges on both QWIPs, have been reported by Yaohul Zhang et al., "A voltage-controlled tunable two-color infrared photodetector using GaAs/AlAs/GaAlAs and GaAs/GaAlAs stacked multiquantum wells", Applied Physics Letters, vol. 68 no. 15 (Apr. 8, 1996) pp. 2114-2116 and by M. Z. Tidrow et al., "A high strain two-stack two-color quantum well infrared photodetector", Applied Physics Letters, vol. 70 no. 7 (Feb. 17, 1997) pp. 859-861. The photodetector of Zhang et al. is tuned alternately to two different infrared wavelength bands, corresponding to the wavelength sensitivity of the two QWIPs, by adjusting the bias voltage applied to the stack. This "voltage tuning" is not entirely satisfactory, however, because each QWIP is sensitive to at least some extent to the other QWIP's band. In the photodetector of Tidrow et al., different pixels are sensitive to different bands, a long wavelength band and a medium wavelength band. All the pixels of the detector are fabricated as tandem stacks, each stack having a long wavelength QWIP above a medium wavelength QWIP; but then the long wavelength QWIPs are etched away from half of the stacks, and separate sets of electrical contacts are provided for the medium wavelength QWIPs thus uncovered and for the remaining long wavelength QWIPs, so that the uncovered medium wavelength QWIPs and the long wavelength QWIPs can be biased separately. Note that in this design, the medium wavelength QWIPs, in the stacks that include both kinds of QWIPs, are inactive. This design suffers from the added fabrication complexity associated with the provision of separate contacts for the active QWIPs and from the need for a non-standard readout-multiplexer unit.
There is thus a widely recognized need for, and it would be highly advantageous to have, a focal plane array, for dual color thermal imaging, with better band discrimination and simpler fabrication than focal plane arrays known heretofore.